The present application relates to a method for manufacturing a microlens and a method for manufacturing a solid-state image sensor provided with the microlens.
Applications of microlenses and microlens arrays are roughly divided into microlenses for condensing light on divided microelements and microlenses alternative to single-part lenses.
In the application to microlenses for condensing light on divided microelements, for example, the microlenses are used for effectively utilizing a quantity of light by condensing incident light on effective regions of photoelectric transducers in a solid-state image sensor (CCD) of a camera including light-receiving elements, or an image sensor for reading faxes.
Also, the microlenses are used for increasing luminance by effectively condensing light in display pixels which are two-dimensionally arranged on a transflective liquid crystal display panel.
In the application to microlenses as alternatives to the single-part lenses, the microlenses are used as alternatives to lenticular lenses of a rear-type projector.
In a general method for manufacturing a microlens (including a microlens of a microlens array), a resist is melted, and a curved surface is formed using the surface tension of the melted resist.
For example, as shown in FIG. 15A, a photoresist film is formed on a lens-forming film 102 formed on a substrate 101, and exposure and development are performed for the photoresist film to form a columnar resist pattern 103.
Then, as shown in FIG. 15B, the resist pattern 103 is fluidized by heating to form the shape of a convex lens. Then, the shape is solidified by cooling to form a convex lens-shaped resist lens pattern 104.
Next, the convex lens-shaped resist lens pattern 104 and the lens-forming film 102 are etched.
As a result, as shown in FIG. 15C, the convex lens-shaped resist lens pattern 104 (refer to FIG. 15B) is transferred to the lens-forming film 102 to form a convex lens-shaped microlens 105.
Alternatively, although not shown in a drawing, a polymer film formed on a substrate is formed into a columnar shape by laser processing, and then the column-shaped polymer film is fluidized to form the shape of a convex lens using the surface tension of the polymer. Then, the polymer is solidified by cooling to form a microlens.
The above-mentioned method for manufacturing a microlens may be applied to the formation of a plurality of microlenses in a large area. However, since the convex lens-shaped resist lens pattern which determines the shape of a microlens is determined by the mobility of the resist, the degree of design freedom is decreased, and it is difficult to achieve desired light diffusion characteristics in pixels.
In addition, when the resist pattern is softened and fluidized by heating, there occurs the problem that if adjacent resist patterns come in contact with each other, the resist patterns are smoothly connected together due to the surface tension, thereby distorting the lens shape. Therefore, it is difficult to form connected microlens arrays.
In a recent compact digital camera or a camera of a cellular phone, miniaturization and thinning have proceeded, and the distance between a lens and a solid-state image sensor (e.g., a CCD or CMOS sensor) has been decreased. Therefore, there has occurred the problem that the angle of light incident on a microlens array is increased in the peripheral region of the solid-state image sensor, thereby causing large deviation from a light-receiving effective region and decreasing sensitivity due to a decrease in light-receiving efficiency.
In order to resolve the problems, a technique is disclosed, in which an optimum lens shape is formed in accordance with the position of a light-receiving portion by an electron beam exposure technique using DML (digital microlens), thereby achieving an improvement in light-receiving efficiency with an incidence angle of 30° or more (refer to, for example, Kimiaki Toshikiyo, Takanori Yogo, Motonari Ishii, Kazuhiro Yamanaka, Toshinobu Matsuno, Kazutoshi Onozawa, Takumi Yamaguchi, “A MOS Image Sensor with Microlenses Built by Sub-Wavelength Patterning”, 2007 ISSCC, SESSION 28, IMAGE SENSORS 28 Aug. 2007).
Further, as a method for changing the shape of each of the lenses in the above-described microlens array, a technique of anisotropically applying pressure to a lens mold is disclosed (refer to, for example, Japanese Unexamined Patent Application Publication No. 9-323353).
The former method has the problem of using a large apparatus for electron beam exposure, and the latter method has the problem of a low degree of freedom for forming a microlens with a desired shape and the difficulty in applying to miniaturization of a microlens.